The invention relates to a power booster of the hydraulic type in which pressurized hydraulic fluid is supplied to the booster and, under the control of the valve mechanism herein claimed, pressure is admitted to a power chamber to act on a booster power piston to actuate the booster. The booster is of the closed center type. It is preferably used as a power booster for a brake system, with the booster actuating at least one hydraulic brake circuit through a master cylinder arrangement.